1. Field of the Invention
The present invention relates generally to the art of turbomachinery blade design and, more specifically, to an optimized blade root attachment profile which achieves a reduction in local peak stress.
2. Description of the Related Art
A turbine has a plurality of rows of stationary and rotary blades. The blades of one row are usually identical to each other and include an airfoil portion and a root portion. The root portion is used to mount the blade in a mounting groove provided in the rotor for rotary blades or in the cylinder for stationary blades.
A common type of root profile for rotary blades is known as the "fir tree" profile, so-called because of the plurality of necks which define a plurality of radially extending lugs.
In the past, fir tree-type blade root contours have been characterized by two symmetrical curvilinear surfaces disposed on opposite sides of the root center line and joined at the bottom by the root bottom and at the top by a lower side of the blade platform.
U.S. Pat. No. 4,191,505, issued to Leonardi, describes a blade profile in which each neck of the blade root has two different radii, with the larger radius being provided in an upper portion of the neck and a smaller radius provided for a lower portion of the neck. This compound contour of the neck in an area where both bending loads and shearing loads act in concert to place the blade material in severe tension is stated to improve low cycle fatigue life, whereby increasing the first radius and decreasing the second radius enables a reduction in maximum stress without a corresponding increase in root depth.
Large rotary blades, such as the last row of blades in a steam turbine experience relatively high peak root/groove stress which results from centrifugal loading. A continuing need exists to minimize this peak root/groove stress, without increasing bearing stress.